In determining the surplus power of a conventional storage battery without measuring the specific gravity of the electrolyte, it is necessary to measure the internal resistance of the battery. There is an existing voltage measuring method on the market, but this involves quite a high error rate. On the other hand, it is possible to measure the internal resistance of a battery using a current meter connected in parallel connected in series with a low-resistance load across the battery for approximating a short-circuit. Such type of measurement not only causes a big spark during operation, but it also causes the temperature of the battery and power cord to rise extremely quickly. The high power consumption is a waste, and it poses a significant danger for non-professionals who attempt it. It would be greatly advantageous to be able to detect a battery's surplus power by correlating the transient charge with internal resistance, which in turn gives an indication of surplus power.